8.2.1Unit 6: Practical Skills in Biology II
Effects of Plant Hormones — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 44 of 48 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Phytochrome and Light
Phytochromes are plant pigments that exist in two interconvertible forms and detect the light environment.
Pr form
- Pr form The inactive form absorbs red light at 660 nm and is converted into Pfr.
Pfr form
- Pfr form The active form absorbs far-red light at 730 nm and is converted back to Pr;
- in darkness Pfr slowly reverts.
Germination
- Germination Red light converts Pr to Pfr and triggers germination, but far-red light reverses the effect.
Flowering
- Flowering Night length alters the level of Pfr, which in long-day plants switches on genes that promote flowering.
Gene expression
- Gene expression Pfr can enter the nucleus and, with PIF3, activate transcription of particular genes.
Part 2 of 5Tropisms and Auxin (IAA)
A tropism is a directional growth response. Indoleacetic acid (IAA) is a type of auxin that controls elongation.
1
When light shines on a shoot from one side, IAA moves from the illuminated side to the shaded side.
2
A higher IAA concentration on the shaded side increases cell wall stretching, so cells there elongate faster.
3
Uneven elongation bends the shoot towards the light, which is positive phototropism.
4
In a horizontal root, IAA moves to the lower side, where a high concentration inhibits elongation.
5
The upper side grows faster than the lower side, so the root bends downwards in positive gravitropism.
Part 3 of 5Gibberellins in Barley Seeds
Gibberellins link water uptake to the mobilisation of food reserves.
1
A dormant barley seed absorbs water when conditions suit germination, and the embryo begins to make gibberellins.
2
Gibberellins diffuse to the aleurone layer and increase transcription of the genes that code for amylase.
3
Amylase hydrolyses starch in the endosperm to maltose, which is converted to glucose.
4
Glucose is transported to the embryo and respired to supply energy for growth.
Part 4 of 5Summary of Plant Growth Factors
Summary of Plant Growth Factors
Auxins
- Auxins They affect apical dominance, root growth and phototropism, mainly by altering gene transcription.
Gibberellins
- Gibberellins They are involved in stem elongation, flowering, seed germination and fruit development.
Cytokinins
- Cytokinins They promote cell growth and cell division in plant tissues.
Abscisic acid
- Abscisic acid It is linked with leaf loss and seed dormancy.
Ethene
- Ethene It is linked with fruit ripening and flowering.
| Growth factor | Example of effect |
|---|---|
| IAA in shoots | Stimulates cell elongation |
| IAA in roots (high concentration) | Inhibits cell elongation |
| Gibberellin | Stimulates amylase synthesis in germinating barley |
Part 5 of 5Exam Checklist & Tips
8.2.1 – Key Points for the Exam
- Tropism A directional growth response of a plant to a stimulus.
- Auxin A group of plant growth factors, including IAA, that influence cell elongation and many other growth processes.
- Phytochrome A light-sensitive plant pigment that exists as Pr and Pfr forms.
- Gibberellin A plant growth factor that stimulates germination, partly by increasing amylase production.
- Apical dominance Suppression of side shoot growth by auxin produced in the shoot tip.
Common Mistakes
- Saying IAA moves to the lighted side of a shoot; it moves to the shaded side.
- Writing that auxin has the same effect on shoots and roots; high IAA inhibits elongation in roots.
- Mixing up the formsred light converts Pr to the active Pfr, and far-red light converts Pfr back.
- Saying amylase is made in the embryo;
- gibberellin from the embryo stimulates its synthesis in the aleurone layer.
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